docs(changelog): move the entry into a changelog.d fragment

Main now takes pending entries as changelog.d/<pr>-<slug>.md
files (#787), so the entry leaves CHANGELOG.md for changelog.d/794-api-method-guard.md.

changelog.d/794-api-method-guard.md

@@ -0,0 +1,7 @@
+- The read-only query proxies state their method contract. `/api/flights`,
+  `/api/military` and `/api/route` answered any verb with the same body
+  while ignoring the request; they now answer `405 Method Not Allowed` to
+  anything but `GET`, the way `/api/geocode` and the other query proxies
+  already did, and declare `Allow: GET` with it the way `/api/radio/stations`
+  does. The refusal precedes rate limiting and any upstream call
+  (daikaginza, #794).